Mass Spectrometry Overview

Isotopes

The radioactive decay of a parent isotope to a daughter isotope changes the isotopic composition of both the parent and daughter elements over time. The rate of such decay is measured in terms of the "half-life" - the time it takes for 50% of a given quantity of a radioactive isotope to decay to its daughter product. 

Some half-lives can be measured in nano-seconds, others in billions of years. The ones that geologists are interested in are usually on the longer side, and include:

  • the beta-decay of 87Rb to 87Sr
  • the alpha-decay of 147Sm to 143Nd
  • the multiple decay series of uranium to lead

These schemes are of interest for two main reasons:

  1. they allow us to date the age of rocks
  2. they allow us to examine and identify heterogeneous parts of the earth such as components within the mantle, or the history of seawater composition, etc.

We also use Ca and B isotopes to look at physical and/or chemical processes which lead to isotopic fractionation in nature.

Analysis

To measure isotopes, we need to take our specimen (rock, water, organic material) and isolate the elements under investigation. This is done by dissolving the sample in acid and using ion-exchange column chromatography to separate out our sample into its component elements. 

To get very high-precision data, we measure isotopic ratios rather than absolute isotopic quantities. We normally look at ratios of the parent or daughter isotope compared to other isotopes of the same element which are not involved in any decay scheme. And to carry out these measurements, to an extremely high degree of precision, we use a mass spectrometer.

Hardware

 

Filament loading

Once we have our element separated, we take it into solution and dry it down onto a filament ribbon made of tantalum (for strontium) or rhenium (all other elements). Each time we run a batch of samples we also load at least one international standard which is the first filament to be run, to check that the machine is operating correctly.

Magazine

Up to 21 filaments can be loaded onto the magazine for each run. The magazine is loaded into the machine's source chamber and pumped down for at least 90 mins; more commonly overnight. We need to obtain a vacuum of less than 3x10-7mbar in the source chamber before proceeding further.

To run a sample, we pass a current through the filament to heat it up to the ionization temperature of the element in question - normally somewhere between 900 and 1900oC. There is a pyrometer on the front of the source chamber which measures the temperature of the filament under analysis.

Focus

Once the filament is heated up, and the sample is ionizing, ions will be emitted from the filament. These need to be focused into a beam which is then fired down the flight tube towards the magnet. To focus and accelerate the beam, we have a series of high-voltage electrostatic extraction and condenser source plates just next to the filament being analysed which can be adjusted to produce an optimum signal.

Magnet

The focused beam of ions is fired, under high vacuum, down the flight tube. This has a bend in it, with a powerful magnet wrapped around the "bend". The magnet deflects and bends the beam of ions, separating it into its constituent isotopes which then continue down the flight tube, each in their own separate beam. Higher masses bend less than lower masses - hence the separation of the isotopes The deflection power of the magnet is adjusted for each element.

Cups

The individual isotope beams are collected in cups at the end of the flight tube. There are 9 cups on this machine - one fixed in the middle (the centre cup) - and 4 adjustable cups on each side. The cups are adjusted to optimal positions to collect the isotopes of the element being analysed. We also have a single secondary electron multiplier (SEM) for samples with very tiny signals - often used for U-Pb dating.

Amplifiers

Every positive ion which hits a cup is neutralised by an electron from the cup's amplifier. This creates an electric current within the amplifier. A typical signal corresponds to approx 0.1 billion ions per second! The amplified signals are digitized and then sent to the computer for you to work with. The amplifiers are calibrated once a day.

Software

The TRITON is software-driven. Initial set-up each day includes setting the magnet, cups, and focus parameters for the element being analysed and doing an amplifier calibration. Once each sample is warmed up, focused and ready to run, a method is chosen to control the way each run is carried out. The standard is run, to check all is well, followed by the samples. It is important that the samples are run in as identical way as possible to the standard. The data file from each run is given a unique name and saved on the computer.